What does seed control in force fields?
Various data that is generated for things like simulations will use random numbers to get their results. In reality, we don't get true random numbers, we get pseudorandom numbers, which is a reproducible way of generating a sequence of numbers that resemble random numbers, with the random seed "defining" the sequence of numbers we get.

For simulations this is helpful as we can use the same random seed to always get the same sequence of numbers. This means every time we play the simulation we see the same result. When we don't like the result we can choose a different seed to get a different result.

The seed in the force field should not change when you alter the size. You should expect the seed to be different for every force field you add. If you don't like the way particles move with the force field you can try using a different seed.